Conventional internal combustion engines produce various pollutants during operation. Generally, most internal combustion engines develop power by burning a hydrocarbon fuel in the presence of air, a mixture of mostly nitrogen and oxygen along with other minor components. During the burning, several exhaust constituents are produced. Some, such as water, are considered rather harmless. Others, such as nitrogen oxides (NOx) are regulated and the production of this pollutant must be controlled. In order to reduce the production of nitrogen oxides, often an exhaust gas recirculation system, hereinafter an EGR system, is provided. In an EGR system, a portion of the exhaust gas from an internal combustion engine is recirculated along a path back into an air intake of the engine. The recirculation of exhaust generally reduces the relative amount of oxygen available for combustion and thus reduces the flame temperature in the engine during combustion. A lower flame temperature greatly reduces the production of nitrogen oxides. Another way to reduce the combustion temperature is to reduce the temperature of the recirculated exhaust. Typically, a cooler is placed in the recirculation path and causes the recirculated exhaust gas to enter the engine at a reduced temperature, thus further reducing the temperature of combustion. Indeed, to reach certain legislative guidelines for emission levels, the exhaust gases must be cooled to some extent.
EGR systems have been used in gasoline engines for at least 30 years and such use is ubiquitous. The use of EGR systems in Diesel engines is more recent. Diesel engines will tolerate more EGR flow than gasoline engines and thus EGR cooling in a Diesel EGR system is important. The coolers in such systems usually have a large heat transfer surface to aid in the transfer of heat from the recirculating exhaust gas to a coolant. Generally, the coolant is introduced behind the heat transfer surface to allow heat to easily pass from the recirculating exhaust gas to the coolant. Unfortunately, during operation of an EGR system, various deposits of soot and other contaminants may accumulate on the heat transfer surface in the cooler and on other conduit portions of the EGR system. The layer of soot will build up in as little as one hundred hours of operation and significantly reduce the ability of the cooler to transfer heat from the recirculating exhaust gas. More specifically, the layer of soot and other contaminants greatly reduces the efficiency of the coolers, thus leading to relatively hot recirculating exhaust gas arriving at the engine intake and reducing the engine's ability to produce power while meeting emissions standards. Such a problem is particularly acute in connection with a Diesel engine.
One approach to this problem has been to employ large coolers. However, the use of large coolers has been considered undesirable because of the high cost and large size. Other approaches have been directed at reducing the amount of deposits. For example, U.S. Patent Application Publication No. 2007/0131207 to Nakamura teaches regulating coolant flow through a cooler based on sensed inlet temperature to reduce deposits. Unfortunately, such a system is based on the principle of increasing the temperature of the recirculating gas. The system is therefore undesirable because it is directly contrary to the concept of reducing the temperature of recirculating exhaust gas to reduce combustion temperature and nitrous oxide production.
Based on the above there is a need in the art for a system designed to control the build up of contaminants in EGR coolers while avoiding the disadvantages set forth above.